ABSTRACT: We measured vertical migration of zooplankton in an arctic fjord at 79°N between June and September 2002 and transcending a period of continuous illumination to one of true day and night to investigate the changing influence of light cues on behavior. Observations made with a moored 300 kHz acoustic Doppler current profiler indicated that two modes of vertical migration occurred during the study period. During the weeks of continuous illumination, backscatter data indicated that there was no net vertical displacement of the population at any time during the 24-h period, but vertical velocity showed a continuous net downward movement in the surface layers and a net upward movement at depth. We interpreted this as unsynchronized vertical migrations by individuals with upward trajectories that slowed closer to the surface and downward trajectories that were most rapid in their initial phases. Synchronized migrations, seen as an upward and downward movement of scattering layers at dusk and dawn respectively, began once true nighttime resumed toward autumn. It is likely that the relative rate of change in light was used as the proximal cue for synchronized migrations. Concurrent net samples identified Calanus finmarchicus and C. glacialis as the most likely contributors to the unsynchronized migration patterns. The high backscatter of the synchronized scattering layers suggests that they included additional taxa such as the euphausiid Thysanoessa spp., the pteropod Limacina helicina, or the hyperiid amphipod Themisto spp.

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